1. Field of the Invention
The present invention relates to a pressure-connection type semiconductor device and more particularly it relates to an improvement in an electrical contact structure between a semiconductor element and a conductor.
2. Description of the Prior Art
In a general pressure-connection type semiconductor device, a semiconductor element and a conductor are electrically brought into contact with each other by mechanical pressure connection. If the semiconductor element and the conductor are welded by a brazing filler metal or the like, the semiconductor device is degraded by thermal fatigue of the brazing filler metal or the like, while no such problem is caused in case of mechanical pressure connection.
However, the following conditions are required for such a pressure-connection type semiconductor device. First, sufficient contact strength must be provided in an interface between the semiconductor element and the conductor. Second, the interface between the semiconductor element and the conductor must have high slidingness and low electrical and thermal resistanoe. Further, no excessive mechanical stress must be applied to the semiconductor element.
In order to cope with such requirements, there has been proposed a semiconductor device which is provided with a cathode sliding compensator between a semiconductor element and a conductor, as disclosed in Japanese patent Publication Gazette No. 4818/1972, for example.
FIG. 1 is an exploded sectional view showing a pressure-connection type semiconductor device employing such a cathode sliding compensator. FIG. 2 is a sectional view of a power thyristor, which is employed as a semiconductor element for the semiconductor device as shown in Fig. 1. Referring to FIG. 2, gallium is first diffused from both surfaces of an N-type silicon substrate 1 to form P-type diffusion regions 2 and 3 of prescribed depth from the surfaces of the N-type silicon substrate respectively, thereby to obtain a PNP structure. Then, phosphorus is diffused in the form of a ring into the P-type diffusion region 3, to form an N-type diffusion region 4. The P-type diffusion region 2 is joined with a molybdenum disc 5 through alloy junction by a brazing filler metal of aluminum. Further, aluminum is vacuum-evaporated on the N-type diffusion region 4 to form a cathode eleotrode layer 7, while aluminum is vacuum-evaporated on a part of the P-type diffusion region 3 to form a gate electrode layer 8. Thus, a thyristor element 9 is formed to serve as a power thyristor.
The structure as shown in FIG. 1 will be make clear by the following description. A cathode sliding compensator 10 is provided on the cathode electrode layer of the thyristor element 9 and an insertion plate 11 is provided on the cathode sliding compensator 10, while a cathode conductor 12 is further provided on the insertion plate 11. On the other hand, another insertion plate 13 is provided under the molybdenum disc 5 of the thyristor eIement 9 and an anode conductor 14 is provided under the insertion plate 13. The cathode conductor 12 and the anode conductor 14 are pressurized from both sides by a housing or the like, thereby to electrically and mechanically connect the respective members with each other.
The cathode sliding compensator 10 is prepared by a metal material such as molybdenum or tungsten, which is similar in thermal expansion coefficient to silicon forming the thyristor element 9. In order to attain excellent slidingness, the surface of the cathode sliding compensator 10 is polished so that roughness of the said surface is not more than 0.5 .mu.m.
The reasons for providing the cathode sliding compensator 1O with surface roughness of not more than 0.5 .mu.m will be make clear by the following description. Assuming that no insertion plate 11 is provided but the cathode conductor 12 is directly in contact with the cathode electrode layer 7 of the thyristor element large friction is caused in the contact surface between the cathode electrode layer 7 and the cathode conductor 12, due to difference in thermal expansion coefficient between the thyristor element 9 and the cathode conductor 12, following temperature change caused by heat generated by the semiconductor device itself in energization. Thus, the cathode electrode layer 7 of the thyristor element 9 is extremely damaged and stress is powerfully applied to the silicon substrate 1. In order to solve the problem, the cathode sliding compensator 10 is prepared by the metal such as molybdenum or tungsten, which is similar in thermal expansion coefficient to silicon forming the thyristor element 9 to be inserted between the thyristor element 9 and the cathode conductor 12. Consequently, sliding between the cathode sliding compensator 10 and the cathode electrode layer or sliding between the cathode sliding compensation 10 and the cathode conductor 12 is facilitated to reduce the friction of the thyristor element g against the cathode conductor 12. This is the reason that both sliding surfaces of the cathode sliding compensator 10 are polished as hereinabove described.
The insertion plates 11 and 13 are prepared by a conductive soft metal such as silver. Such insertion plates 11 and 13 are provided for the following reason: In the thyristor element 9, the p-type diffusion region 2 of silicon is in alloy junction with the molybdenum disc 5 through the brazing filler metal 6 of aluminum. As the result, the thyristor element 9 is curved in a constant direction following heat generated in energization thereof, due to difference in thermal expansion coefficient between silicon and molybdenum. Thus, assuming that the molybdenum disc and the cathode electrode layer are directly pressure-connected with the anode conductor 14 and the cathode conductor 12 respectively in such a curved state of the thyristor element 9, no sufficient contact can be obtained between such elements. Thus, the conductive soft metal members are interposed between such elements, to ensure sufficient contact therebetween.
Since aluminum is easily oxidized, an extremely thin aluminum oxide film is formed on the surface of the cathode electrode layer in the process of manufacturing the semiconductor device. ConsequentIy, electrical contact between the cathode electrode layer and the cathode sliding compensator 10 is made insufficient by presence of such an aluminum oxide film, to cause fall of potential. Thus, power loss in the thyristor element 9 is increased to exert a bad influence on thyristor characteristics.